81 threads does seem very high, unless you had a lot of windows or lengthy commands running at the time.
I wonder if something is going wrong as each thread tries to shut down, maybe as it detaches from a DLL that's been loaded into the Opus process, and the threads are piling up until some kind of limit is reached?
When it happens again, try selecting some of the threads, particularly the "endthreadex" ones but also some of the others, and click the Stack button to see what sort of stuff they're running. Maybe it'll point to a 3rd party DLL that seems common to all those apparently-hung threads. I think that sort of info is contained in the MiniDumps as well so GPSoftware may be able to access it if you send the dumps their way.
If it was something you could reproduce quickly then I'd say it'd be worth trying out the general slowdown/instability tests, especially disabling anti-virus (since that will hook into every thread), but most of those changes/tests aren't ones you'd wan to leave running for an extended period of time. (e.g. Turning off a/v is fine to test something but turning it off all day is another thing!)